1. Field of the Invention
The present invention is directed to a method and apparatus of detecting tool abnormality in a machine tool and more specifically to a method and apparatus whereby tool wear and damage can be detected on the basis of electric current measured in the real cutting section of a machine tool.
2. Description of the Prior Art
As is generally known in the art, conventional machine tools for use in cutting or machining a workpiece are provided with one or more servomotor that serves to feed the workpiece and a spindle that holds a tool adapted to cut the workpiece into a desired configuration.
The machine tool carries out the cutting operation while the workpiece is fed along a predetermined path by means of the servomotor. If there occurs such abnormality as impermissible degree of wear, the cutting operation will not proceed smoothly and therefore the machine tool will suffer from overload.
In the past, the overload condition was determined by use of an external sensor or a computerized numerical controller that can detect load-dependent current value of the spindle motor and the servomotor. In other words, the overload condition was detected by determining whether the current value supplied to the spindle motor or the servomotor exceeds a preselected maximum reference value, under the assumption that the overload condition shall necessarily take place at the occurrence of tool abnormality.
At this time, in order that the overload detection should be accomplished only at the real cutting section, the load applied to the spindle motor is detected for the section other than accelerating and decelerating sections where the workpiece is simply fed, and the load applied to the servomotor is detected for the section other than a prompt feeding section and accelerating and decelerating sections.
With the prior art load detecting method described above, however, it is difficult to figure out a variety of below-listed unusual loads that may occur at the time of the tool being in an abnormal condition.
First of all, there may be such instances that the load current detected is smaller in the real cutting section than in the load-free feeding section, depending on the workpiece cutting direction. For example, the load current may be greater at the time of non-cutting operation than the real cutting operation while a tool post is displaced in a horizontal lathe. This is because the tool post is of heavyweight nature and requires greater current to move than at the real cutting time due to the influence of the gravity. Moreover, the load applied to the servomotor is not proportional to the cutting load unlike the load applied to the spindle motor mainly because the load of the servomotor may vary with the posture of a tool during the cutting operation and the shape of the workpiece to be cut. This means that it is no longer possible to determine a stereotyped load pattern in case of multi-axis cutting process.
Secondly, the cutting tools employed in a machine tool is usually provided with plural numbers of cutting edges and, therefore, irregular oscillatory load current may be generated in case that one or two cutting edge of the tool is severely worn or broken. The oscillatory load current at this time becomes either greater or smaller than the reference load current during normal cutting operation. No tool abnormality can be detected in the event that the oscillatory load current is smaller than the reference load current.
Thirdly, non-load condition may take place even in the real cutting process if the tool is broken completely and fails to make contact with the workpiece, although overload condition would be kept at the time of severe wear or partial damage of the tool. At this instance, it becomes impossible to detect the tool breakage by the conventional method wherein determination is made as to whether the current supplied to the spindle motor or the servomotor exceeds a maximum reference value. For the very reason, the cutting depth of the tool in the next cutting process is increased beyond a permissible limit, which leads to attendant breakage of the tool newly changed in the subsequent cutting process.